1. Field of the Invention
The present invention relates to a substrate grinding method and device for grinding a subject surface of a substrate by pressing a grinding tool such as a grinding wheel on the subject surface as rotating the grinding tool.
2. Description of the Related Art
A reduction in size and weight of a semiconductor device has increasingly become remarkable in recent years. To realize a reduction in thickness of the device, the back side of a device substrate such as a semiconductor wafer having a front side formed with many devices is ground to obtain a desired thickness prior to separating the device substrate into the individual devices. In general, the back side of the substrate is ground by using a grinding device including a vacuum chuck type chuck table for holding the substrate by using suction vacuum and a grinding tool such as a grinding wheel opposed to the chuck table, wherein the chuck table holding the wafer is rotated and the grinding tool is rotated and fed to be pressed on the subject surface of the substrate. When the substrate thickness has reached a desired value in grinding the substrate by using such a grinding device, so-called “spark-out” is performed to stop the feed of the grinding tool and continue the rotation of the grinding tool at the feed stop position for a predetermined period of time. After the elapse of the predetermined period of time for the spark-out, the grinding tool is retracted from the substrate.
The spark-out is required for the purposes of flattening the ground surface of the substrate by at least once rotating the substrate and removing a grinding strain from the substrate. During grinding of the substrate, the grinding tool may be contracted by application of a working load or physical distortion may be produced between the chuck table and the grinding tool, causing the inclusion of residual stress in the grinding device. Accordingly, in performing the spark-out, the residual stress is relieved, so that the grinding tool is engaged in the ground surface of the substrate. As a result, the ground surface of the substrate is excessively ground and the substrate thickness therefore becomes smaller than the desired value. To cope with this problem, the feed stop position of the grinding tool, or the spark-out start position is set slightly higher than the position corresponding to the desired thickness in expectation of the amount of inertial grinding occurring in performing the spark-out. Thus, the amount of inertial grinding is provided as a correction value.
However, even though the grinding conditions are fixed, the amount of inertial grinding caused by the residual stress changes according to various factors such as a minute change in condition of the grinding tool that may be worn and a difference in machinability due to variations in thermal history of the substrate. On the other hand, the correction value corresponding to the amount of inertial grinding is a fixed value. Accordingly, there is a case that the correction value does not become a proper correction value. As a result, the substrate thickness after finishing of the grinding, i.e., after the spark-out in grinding numerous substrates may vary. That is, there are variations in substrate thickness after the spark-out in grinding numerous substrates.